Conventionally, in many drive axles for farm tractors or the like that require a large steering angle, the differential gear output shaft is coupled to the wheels via a constant-velocity type double Cardan joint. While a double Cardan joint requires a long axial length and a large outer diameter, it can have an intersection angle exceeding 50°. The constant-velocity type double Cardan joint is ordinarily lubricated by grease that has been filled in an axle housing. In the meantime, a drive axle of a large vehicle such as a rough-terrain crane vehicle adapted to on-road driving uses one constant velocity universal joint of Rzeppa-type or ball-fixed type (hereinafter sometimes referred to as “BJ” or “BJ-type”) with bellows boot to simplify the mechanism (see Patent Document 1). The BJ type includes an outer ring on which a curved track groove is formed in an axial direction on a spherical inner diameter surface, an inner ring on which a curved track groove is formed in an axial direction on a spherical outer diameter surface, a plurality of balls for torque transmission disposed in ball tracks that are formed through coordination between the track groove of the outer ring and the track groove of the inner ring corresponding thereto, and a cage provided with pockets for holding the balls.
When a constant velocity universal joint of BJ-type is used for a drive shaft, an axle section (driven shaft) that integrally extends from one end of the outer ring in an axial direction is coupled to a wheel bearing, and a shaft (drive shaft) that is spline engaged with a shaft hole of the inner ring is coupled to a slide-type constant velocity universal joint. When there is an angular displacement between the two axes, that is, between an axle section of the outer ring and a shaft of the inner ring, the balls housed in the pockets of the cage are always held in an angle bisecting plane of any operating angle, whereby constant velocity of the joint can be maintained. The operating angle herein refers to an angle created by the axle section of the outer ring and the shaft of the inner ring.
In the meantime, a drive axle of a large vehicle such as a rough-terrain crane vehicle includes, in a section inside of the wheel hub, a so-called hub reduction including a reducer such as a planetary gear mechanism to prevent large drive torque from acting on the BJ-type constant velocity universal joint.    Patent Document 1: Japanese Patent Laid-open Publication No. Heisei 4-358970